A cellular RF communications cell consists of a plurality of low power transmitters, each permanently coupled to an antenna. The antennas are arranged such that each radiates from the center of the cell in 60.degree. sectors, thereby forming a hexagonal shaped cell. A plurality of these cells are combined to form a cellular RF communications system. The system is linked to landline telephone networks allowing mobile traffic to communicate with the telephone networks through the cellular system.
The landline telephone networks can communicate with mobile traffic using either Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM) techniques. Using TDM, it is possible to map a plurality of traffic channels onto each transmitter's carrier by dividing the carrier into a plurality of time slots. This allows a greater number of channels to be transmitted in a given time than would be allowed if only one channel was transmitted per carrier. TDM, therefore, increases the efficiency of the cell's frequencies. TDM is now being introduced into cellular systems. Using FDM, a plurality of transmitters can simultaneously transmit signals of different frequencies from the same antenna without one signal interfering with another. FDM is commonly employed in the cellular systems at the present.
Cellular systems achieve large traffic handling capabilities through the reuse of frequencies in a number of cell sites within the same metropolitan area. The low transmission power and the placement of the antennas allows the quality of the speech signal to remain above an acceptable threshold. This threshold is referred to in the art as the Carrier to Interference ratio (C/I). C/I is a ratio of the signal strength of the received desired carrier to the signal strength of the received interfering carriers. A number of physical factors can effect C/I in cellular systems: buildings, geography, antenna radiation patterns, mobile traffic transmitting power, and mobile traffic location within the cell.
Presently, a four cell reuse pattern in one of the densest frequency reuse patterns which produces an acceptable C/I. U.S. Pat. No. 4,128,740 to Graziano for an antenna array for a cellular RF communications system describes such a four cell frequency reuse pattern. While the four cell frequency reuse pattern permits closer reuse of the co-channel frequencies and thus provides higher traffic capacity, it introduces an inefficiency in that the total number of channels available to the cellular system are divided into more groups of frequencies. This partitioning results in fewer channels available in a given sector and can lead to situations where specific sectors cannot handle the traffic.
Cellular systems are being used with increasing regularity by both business and personal mobile traffic. This increased utilization has created a problem with frequency use in large metropolitan areas. When a fixed sector has more mobile traffic than available frequencies, the sector must ignore the excess traffic until a frequency is free. Due to both frequency limitations by relevant government authorities and the four cell reuse pattern, new frequencies cannot always be added to the cell to alleviate this problem. There exists a need, therefore, for a way to improve traffic handling capabilities of cellular communications sites.